The detection of CO2 is essential for a range of applications including reduction of false fire alarms, environmental monitoring, and engine emission monitoring. For example, traditional smoke detectors monitoring particles can have false fire alarm rates as high as 1 in 200 in aircraft applications. Alternatively, monitoring the change of CO and CO2 concentrations and their ratio (CO/CO2) can be used to detect the chemical signature of a fire. Electrochemical CO2 sensors which use super ion conductors (such as Na Super Ionic Conductor or NASICON) as the solid electrolyte, and auxiliary electrolytes (such as Na2CO3/BaCO3) have great potential for in-situ fire detection and other applications. In recent years, there has been a significant effort to develop bulk and miniaturized electrochemical CO2 sensors. Compared to bulk material and thick film solid electrolyte CO2 sensors, miniaturized sensors fabricated by microfabrication techniques generally have the advantages of small size, light weight, low power consumption, and batch fabrication.
Four factors are typically cited as relevant in determining whether a chemical sensor can meet the needs of an application, namely, sensitivity, selectivity, response time and stability. Sensitivity refers to the ability of the sensor to detect the desired chemical species in the range of interest. Selectivity refers to the ability of the sensor to detect the species of interest in the presence of interfering gases which also can produce a sensor response. Response time refers to the time it takes for the sensor to provide a meaningful signal. By meaningful signal it is meant that the signal has reached, for example, 90% of the steady state signal when the chemical environment experiences a step change. Stability refers to the degree which the sensor baseline and response are the same over time. It is desirable to use a sensor that will accurately determine the species of interest in a given environment with a response large and rapid enough to be of use in the application and whose response does not significantly drift over its operational lifetime.
Current bulk or thick film solid electrolyte carbon dioxide sensors have the disadvantages of being large in size, high in power consumption, difficult in batch fabrication, and high in cost. The carbon dioxide sensor design described herein has the advantage of being simple to batch fabricate, small in size, low in power consumption, easy to use, and fast responding.
FIG. 1A is a cross-sectional schematic illustration 100 of a prior art bulk carbon dioxide gas sensor. Referring to FIG. 1A, reference numeral 101 is an electrolyte known as NASICON which is an acronym or partial acronym for Na3Zr2Si2PO12 and is oriented between a platinum (paste) 103 and a Sodium Carbonate/Barium Carbonate (Na2CO3/BaCO3) layer 102. A reference electrode 105 engages the platinum paste and a gold working electrode 104 resides in contact with the interface of the Sodium Carbonate and/or Barium Carbonate (Na2CO3/BaCO3) 102 and the NASICON 101. By Sodium Carbonate and/or Barium Carbonate (Na2CO3/BaCO3), it is meant that either Sodium Carbonate (Na2CO3) or Barium Carbonate (BaCO3), or their mixtures may be used. The sensor is supported by quartz glass tubes (insulators) 106 for reference gases.
FIG. 1 is a cross-sectional schematic illustration 100 of a prior art gas sensor disclosing an Alumina substrate 107, interdigitated Platinum metal electrodes 108, a first solid electrolyte, NASICON 109, between the electrodes, and Sodium Carbonate and/or Barium Carbonate (Na2CO3/BaCO3) 110 covering the NASICON and the electrodes. The first solid electrolyte is selected from the group consisting of NASICON, LISICON, KSICON, and β″-Alumina (beta prime-prime alumina in which when prepared as an electrolyte is complexed with a mobile ion selected from the group consisting of Na+, K+, Li+, Ag+, H+, Pb2+, Sr2+ or Ba2+). By Sodium Carbonate and/or Barium Carbonate (Na2CO3/BaCO3), it is meant that either a material containing Sodium Carbonate (Na2CO3), Barium Carbonate (BaCO3), or a mixture of Sodium Carbonate and Barium Carbonate may be used. An important feature of electrochemical cells of this type are the three-contact boundaries seen in 100. It is the intersection of 108, 109, and 110. These contacts significantly determine the effectiveness of the sensor and their number and surface area should be maximized. The inventors of the instant patent application disclosed this structure in a conference in Lisbon, Portugal in 2004 and this structure was illustrated or described in an FAA website thereafter. This structure is a schematic and not ideally achievable for a number of reasons. First, to obtain the structure exactly as illustrated in FIG. 1 a perfectly sized and aligned mask is necessary. In other words the width of the mask and its apertures has to be absolutely perfect and the alignment has to be absolutely perfect to achieve uniform three-point contact along the joint of the metal electrodes, NASICON and Sodium Carbonate/Barium Carbonate (Na2CO3/BaCO3). Statistically, given manufacturing tolerances the structure depicted in FIG. 1 is very difficult to achieve. Photolithographic masks are aligned by hand with the aid of an electron microscope. Any misalignment of the photolithographic mask will result in photoresist trapped between NASICON and electrode finger and therefore result in a failed sensor. Simply put, the structure of FIG. 1 is very difficult to manufacture exactly as shown. Errors in manufacturing probably will result in a failed structure such as that depicted in FIG. 5D. One of the innovations of the instant invention is to realize the advantages of not having to perfectly duplicate the structure of FIG. 1, which represents the structure obtained using standard procedures of microfabrication engineers.
Previously, most solid electrolyte CO2 sensors developed were bulk sized or thick film based as illustrated in FIG. 1A, which involves complicated fabrication process of hot press or screen printing. The power consumption of these sensors is very high and batch fabrication is very difficult. Porous electrodes are typical: Electrodes formed by the thick film technique are not sufficiently porous. Using a non-porous electrode can lead to the formation of sodium carbonate Na2CO3 which hinders the working electrode. The formation and dissociation of sodium carbonate Na2CO3 at the electrodes results in slower response time.
Most often (in the prior art) two sensing materials were used in a solid electrolyte CO2 sensor structure. In the effort to miniaturize a CO2 sensor, the standard approach was to first deposit one sensing electrolyte on the substrate, the electrodes were then deposited on top of the electrolyte, and finally the auxiliary electrolyte was deposited on the electrodes. Humidity, liquid chemical processing, and/or physical vibration tends to erode or loosen the electrolyte underneath the electrodes. This structure limited the application of standard microprocessing techniques one might employ such as photolithography. These properties limited the miniaturization of the sensor using this structure, because the electrodes could only be deposited by a shadow mask, which usually produces electrodes with less integrity when the feature is very small. That is one reason few stable and functional miniaturized sensors of this type exist.
Photolithography is used in device fabrication processes every time a pattern is transferred to a surface. It allows ion implantation or etching of a material in selected areas on the wafer (substrate). Photoresist is a photosensitive organic substance which is a sticky liquid with high viscosity which is typically spun onto a wafer and then thermally hardened in an oven. Photoresist may be positive or negative. When positive photoresist is exposed to light it breaks down long-chain organic molecules into shorter chain molecules which can be dissolved by a chemical solution called a developer. When negative photoresist is exposed to light it induces cross-linking of organic molecules such that a high atomic mass is achieved by producing longer-chain molecules. In the example of longer chain molecules, an appropriate developer solution is then used to remove the resist that has not been exposed to light. The transfer of the desired patterns onto the photoresist is made using ultraviolet light exposure through a mask which is typically a quartz plate. Masks are used in two modes. Contact lithography involves overlaying the mask directly into contact with the photoresist and proximity photolithography involves spacing the mask a distance above the photoresist. The use of photolithography enables miniaturization, batch processing, and more exact duplication of a given sensor structure. Employing these techniques can fundamentally change and improve the sensors produced; a significant technical challenge is to apply these techniques for some material systems such as those used for CO2 sensor production.